A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium for use in various kinds of magnetic recording apparatuses. More particularly, the invention relates to a perpendicular magnetic recording medium for use in a hard disk drive used as an external storage device of, for example, a computer or an AV apparatus and is capable of achieving high-density magnetic recording.
B. Description of the Related Art
In recent years, a magnetic recording medium for use in a hard disk drive (HDD) has employed a perpendicular magnetic recording method in order to further improve recording density.
The perpendicular magnetic recording medium mainly includes a magnetic recording layer made of a hard magnetic material, a seed layer that aligns the magnetic recording layer in a desired direction, a protective layer that protects the surface of the magnetic recording layer, and an underlayer that is made of a soft magnetic material and concentrates a magnetic flux generated by a magnetic head used to record information on the magnetic recording layer.
In the perpendicular magnetic recording method, a recording bit recorded on the perpendicular magnetic recording medium is affected by the diamagnetic field of adjacent recording bits such that the stability of the magnitude of remnant magnetization is stabilized as recording density increases. As a result, the perpendicular magnetic recording medium can have high thermal fluctuation resistance.
In addition, the perpendicular magnetic recording medium includes a soft magnetic underlayer that is made of a soft magnetic material and is provided between a substrate and the magnetic recording layer. In this way, the soft magnetic underlayer sharply draws the magnetic field generated from the magnetic head. Therefore, the magnetic field gradient is reduced and the influence of the write spreading of signals is also reduced.
In order to further improve the recording density of the perpendicular magnetic recording medium, for example, it is necessary to reduce the size of a magnetic crystal grain, improve the separation performance of the magnetic crystal grain, and reduce the orientational dispersion Δθ50 of the c-axis which is the magnetization easy axis of the magnetic layer. The following techniques are disclosed as a means for satisfying the above mentioned conditions.
Japanese Patent Application Laid-Open (JP-A) No. 2008-34060 discloses a technique in which an orientation control layer provided below a Ru intermediate layer is made of a non-magnetic material having NiCr or NiCu as a main component to reduce the orientational dispersion Δθ50. JP-A No. 2010-44842 discloses a technique in which a CuTi seed layer is used as an orientation control layer to reduce the orientational dispersion Δθ50 even when the thickness of the layer is small. JP-A No. 2002-358617 discloses a technique in which an intermediate layer provided below a magnetic layer is made of NiFeCr, thereby improving the orientation of the magnetic recording layer. JP-A No. 2008-84413 discloses an example in which a seed layer is made of FeCoB as a material other than the above-mentioned material.
As such, many techniques for improving the recording density of the perpendicular magnetic recording medium have been proposed. However, the characteristics of the perpendicular magnetic recording medium depend on various conditions, such as the component and composition of each layer to be laminated and the order in which the layers are laminated. In the techniques according to the prior art, all of the above-mentioned conditions are not optimized, and the medium characteristics have both the advantages and the disadvantages. Therefore, in recent years, there is a demand for further improvement in the characteristics of the perpendicular magnetic recording medium.
In order to increase the signal output of the perpendicular magnetic recording medium and reduce its noise, thereby increasing the S/N ratio, it is necessary to minimize the orientational dispersion of the magnetic recording layer. In addition, in order to reduce the noise of the magnetic recording medium, it is necessary to reduce the crystal grain size of the magnetic recording layer.
The seed layer or the intermediate layer has a function of controlling, for example, the crystallinity, orientation, and crystal grain size of the magnetic recording layer formed thereon, and it has been known that the seed layer or the intermediate layer has an influence on the characteristics of the magnetic recording layer. Therefore, in order to reduce the crystal grain size of a magnetic recording layer material, it is effective to reduce the crystal grain size of the seed layer or the intermediate layer. However, it has been known that, when the thickness of the seed layer or the intermediate layer is reduced, the crystal orientation of the magnetic recording layer material is deteriorated, the magnetic separation between the magnetic crystal grains is hindered, and the magnetic characteristics of the magnetic recording layer are deteriorated. When considering the above-mentioned points, it is necessary to reduce the thickness of the seed layer or the intermediate layer while maintaining or improving the magnetic characteristics of the magnetic recording layer, not simply reducing the thickness of the seed layer or the intermediate layer.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.